For many years seismic exploration for oil and gas has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected point on a terrain, or another energy source having capacity for delivering a series of impacts or mechanical vibrations to the earths surface. Offshore, air gun sources and hydrophone receivers are commonly used. The acoustic waves generated in the earth by these sources are transmitted back from strata boundaries and/or other discontinuities and reach the earth's surface at varying intervals of time, depending on the distance traversed and the characteristics of the subsurface traversed. On land these returning waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical analog signals, which are generally referred to as traces. In use on land an array of geophones is laid out along a grid covering an area of interest to form a group of spaced apart observation stations within a desired locality to enable construction of three dimensional (3D) views of reflector positions over wide areas. The source, which is offset a desired distance from the geophones, injects acoustic signals into the earth, and the detected signals at each geophone in the array are recorded for later processing using digital computers, where the analog data is generally quantized as digital sample points, e.g., one sample every two milliseconds, such that each sample point may be operated on individually. Accordingly, continuously recorded seismic field traces are reduced to vertical cross sections and/or horizontal map views which approximate subsurface structure. The geophone array is then moved along to a new position and the process is repeated to provide a seismic survey.
A seismic data processing technique referred to herein as delta amplitude dip (DAD) accentuates areas of waveform tuning in hydrocarbon filled porous formations, and is well suited for directly indicating the presence of hydrocarbons in those hydrocarbon containing formations. This DAD technique is disclosed in U.S. Pat. No. 5,543,958 issued to Dennis B. Neff, and the entire disclosure of this patent is incorporated herein by reference. According to the DAD approach, an attribute of a subsurface reflection point is determined from the delta amplitude in the direction of maximum dip, normalized by the amount of dip. This DAD value of a seismic attribute is derived from traces obtained from multipoint coverage of a dipping subsurface interface, and is used to identify the presence of hydrocarbons in the subsurface formations. While this DAD technique is considered to be a significant exploration and exploitation tool, it requires a preprocessing step of manually locating and picking horizons, and accordingly elimination of the preprocessing step so as to achieve a more fully automated DAD process would be highly desirable.
Also, it is well known by persons skilled in the art of seismic prospecting that the compressional P-wave reflection coefficient at an interface separating two media varies with the angle of incidence of seismic energy. A processing technique referred to as amplitude versus offset (AVO) is well known by those skilled in the art for relating the reflected amplitude variation to the presence of hydrocarbon accumulations in a subsurface formation. According to the AVO approach, attributes of a subsurface interface are determined from the dependence of the detected amplitude of seismic reflections on the angle of incidence of the seismic energy. This AVO approach determines both a normal incidence coefficient of seismic reflection, and a gradient component of seismic reflection, and the cross plotting of normal incidence amplitude and gradient data is often used in the method for identifying hydrocarbons. In an AVO processing technique, one derives the amplitude R of a reflected seismic wave from an interface as a function of the angle of incidence .theta. from the normal according to the equation: EQU R.sub.(.theta.) =A+B sin.sup.2 .theta.
In this equation, the coefficient A is the normal incidence coefficient, and the coefficient B is the gradient component, which is representative of the rate of change of amplitude with the square of the sine of the angle of incidence.
AVO analysis and processing as an exploration tool for risk analysis has been significantly advanced in the last five years through better processing and presentation schemes. Accordingly, certain indicators derived from AVO analysis, such as using the positive A*B product as a direct indicator of hydrocarbons, have been successful in identifying the location of many gas and oil reservoirs. While using such indicators, however, many valid hydrocarbon AVO anomalies, which may be indicators of hydrocarbon, are overlooked because they are associated with medium or hard sand layers that do not and should not have a higher amplitude reflection in the far offsets. Also, false bright spots often remain after AVO processing. Particularly problematic in AVO processing are the medium porosity, or so called Class II sands, which frequently reverse polarity with greater offset when gaseous hydrocarbons are present in the formation.
In conventional DAD or AVO processing, multiple seismic traces are collected from source receiver pairs having different offsets and thus multiple angles of incident seismic energy, and where the collected signal traces are each reflected from a common subterranean reflection point. Such a group of traces is referred to as a common depth point (CDP) gather. Typically, seismic reflection points are midpoints between the source and receiver pair for various offsets, and as such this gather is also often referred to as a common midpoint (CMP) gather.
Accordingly it is an object of this invention to extract more useful subsurface information from seismic amplitude data without requiring information regarding actual properties of the rock.
It is a more specific object of this invention to more consistently distinguish sands and porous carbonates with hydrocarbon from surrounding formations.
Another more specific object of this invention is to better image the edges of hydrocarbon-bearing reservoirs.
It is a still further object of this invention to provide a method and system for improved processing of seismic data that is compatible with previously implemented AVO analysis techniques.